Oxide coatings on metals



R. L. EVERY OXIDE COATINGS ON METALS 4 Sheets-Sheet Filed Feb. 10, 1966E LD Mm T LC Ew L D E O R T C E L RE T 5 E MT 0 M Du E m FIGURE IPOTASSIUM CHLORIDE SOLUTION 54 FIGURE 3 INVENTOR. v RICHARD L. EVERY Y2& 26 24M E m A E S R W W G 0 P mm 1 N 0 Sm a la SR M CT.% Mm A L 0 I mv m TN E NOD F EC E M NC W A O P P F m A 2 o E @E N V m m m T Fr. 5 A 16w M R 0 m 0 0 M AMQOKPQMJM mozmmmumm 0.: w .r0

AT TORNEJ APW'E 3%? R. EVERY OXIDE COATINGS 0N METALS 4 Sheets-Sheet 2Filed Feb. 10, 1966 O 0 O .m Q mjmoz 2.82 3.2

FIGURE 4 0 w w 0 m 0 R O 9 U m L O E 8 m 5 m m E m m 2 m MW 6 P I v F PO 5 O 4 O 3 C m 0 O 0 O 2 3 9 5 l 4 3 3 3 O M A i! M mJmoz m.:o

9 GOLD GOLD OXIDE TEMPERATURE APPROXIMATELY 25C INVENTOR. RICHARD L.EVERY PER CENT SULFURIC ACID FIGURE 6 ATTORNE? Ami! H, 1967 R. L. EVERY3,3El3,659

OXIDE COATINGS ON METALS Filed Feb. 10, 1966 4 Sheets-Sheet 15 m" +540 ng 5 Z 2 +500 l 2 +420 FI-PIO IN 85% H3PO4NO Fe 20C 30 40 50 6O 70 I so90 I IIO I20 I30C f 90 2 a o g z 130 J I70 Pt-PtO IN NaOH N0 Fe 20C 504o 50 e0 7O 50 90 no I20 I30C FIGURE 8 h+5l0 33 (D 5 2 +470 2 o PI-PIOIN 67% H2804 NO Fe AT |OOC x PI-PIO IN 57% H so NO Fe AT C g +450 2 2 40 I0 20 so a0 90 I00 340 HOURS +450 m 5 PI-PIo IN H3PO4 N0 Fe AT 25C 5 2z +4I0 2 0 IO 20 3O 4O 5O 6O 70 6O I00 HOURS FIGURE IO INVENTOR.

RICHARD L. EVERY BY 94. l I

ATTORNEY Apm'fi M, 19637 R. L. EVERY 3,313,659

OXIDE COATINGS ON METALS Filed Feb. 10, 1966 4 Sheets-Sheet 4 5 Ed +Ieo0O 2 :1: H200 ANODIC POLARIZATION CURVE FOR= 5 I020 MILD STEEL IN NIJHSO4o L (MOLTEN) AT 42OF B j 800 E LL] Q E Q 400 (DATA POINTS TAKEN AT 2MIN. INTERVALS) 5 E F 5 [I 0 O i- 400 LU 2 000 A ODIC CURREN oI I0 I00I000 I0oo.o

CURRENT DENSITY(MA/lN.

FIGURE I! m +2000 CD 3 I600 9 T 5 g- I200 O O N E j m 800 5-3 ANooIcPOLARIZATION CURVE F0R= E o 304 STAINLESS STEEL IN NcIHSO4 400 (MOLTEN)AT 420F CL E 3. i2 0 g I (0ATA POINTS TAKEN AT 2 MIN. INTERVALS) gCATHODI DENSITY O 800 0| I0 I00 IO0.0 1000.0

CURRENT DENSITY (MA/IN? FIGURE I2 INVENTOR.

RICHARD L. EVERY A TTORNE'Y United States Patent 3,313,659 GXIDECOATHQGS 6N METALS Richard 1.. Every, Ponca City, Okla, assignor toContinental Oil Company, Ponca City, Okla, a corporation of DelawareFiled Feb. 19, 1966, Ser. No. 527,381 Claims. (Cl. 148-611) Thisapplication is a continuation-in-part of application for United StatesLetters Patent Ser. No. 368,956 filed May 20, 1964, now abandoned whichwas a continuation in part of application for United States LettersPatent Ser. No. 260,804 filed Feb. 25, 1963 now abandoned. The inventionrelates to a method of putting an oxide coating on a metal. Thisinvention also relates to a method of making metal-metal oxideelectrodes. The electrodes produced by this method are useful asreference electrodes in the determination of the susceptibility of ametal to attack by corrosive electrolytic solutions. Such methods ofdetermining the susceptibility of a metal to attack by corrosiveelectrolyte solution form the subject matter of our co-pendingapplication No. 368,956 mentioned above.

In the anodic polarization systems known to me to be in commercial useat the present time, two types of electrochemical half-cells aregenerally employed as the reference electrode. Of these the mostfrequently used is the calomel half-cell, an electrode accepted as astandard and widely used in other applications by virtue of itsrelatively stable and constant potential. The other reference electrodewhich is used in the anodic polarization corrosion system is asilver-silver chloride electrode. The calomel electrode is a liquidelectrode while the silver-silver chloride electrode is solid.

Although corrosion control systems employing these two types ofreference electrodes to determine corrosion susceptibility havegenerally worked satisfactorily under certain preselected conditions,these electrodes lack universality with respect to the types ofelectrolytes in which they may be used; and each suffers from seriouslimitations when subjected to variations in the concentration of theelectrolyte in which it is used or to varying temperature conditions.

The silver-silver chloride half-cell is rather difficult to prepareproperly and because of its softness, presents some difiiculty inmounting. The most serious shortcomings of this reference electrode,however, are its solubility in oleum, a frequently encounteredelectrolyte in such corrosion protection systems, and its susceptibilityto severe erosion in highly agitated systems.

Although the calomel electrode is of more universal utility than thesilver-silver chloride electrode, in several respects it presentsserious disadvantages not shared by the silver-silver chlorideelectrode. For the most part, these stem from the fact that it is aliquid electrode; thus to prevent contamination of the electrode, asuitable electrochemical salt bridge must be utilized to provide aconductive path between the corrosive electrolyte and the calomelelectrode. Not only are reference electrode assemblies employing suchsalt bridges difficult to install but they also present a problem ofcontamination of the electrolyte product by the salt solution of thebridge; moreover, where large tanks filled with a corrosive electrolyticchemical are to be protected, the salt bridge must be of considerablelength; and for this reason, it is rather fragile and is, therefore,subject to mechanical malfunction particularly in systems in which thecorrosive electrolyte is violently agitated. Also, in product storagesystems where the corrosive electrolyte is maintained under pressure,some means must be provided for equalizing the pressure acting upon thesalt solution and the electrolytic bridge.

Patented Apr. 11, 1967 The ideal reference electrode for use indetermining susceptibility of corrosive attack must possess certaincharacteristics. The properties of the ideal reference electrode for usein anodic polarization corrosion control systems may be summarized asfollows:

(a) Insolubility in any electrolyte in which it may be used and in anyconcentration of such electrolyte;

(b) That it exhibit an electrical potential which is essentiallyindependent of electrolyte concentration and temperature;

(c) That it does not require use of an electrolytic salt bridge; and

(d) That it can be fabricated in the form of a durable structure whichwill resist erosion, abrasion and impact.

In addition to the use as electrodes of metal-metal oxides prepared bythis method, other uses have been found for the oxide coated metals.Exemplary or other uses of the oxidized metals are: use in batteries andfuel cells, as elements in electron emission, as electrolyticrectifiers, capacitors, use in catalysis, in electrcchemistry and as aprotection of the metal from corrosion and erosion.

An object of the present invention is to provide an improved electrodeand methods of making the same which will have properties closelyapproaching the abovementioned properties of the ideal referenceelectrode.

Another object of the invention is to provide new reference electrodesfor use'in determining the susceptibility of metals to corrosive attack,which electrodes may be installed and used in anodic polarizationcorrosion control systems by being placed directly in contact with thecorrosive electrolyte and without requiring the use of a salt bridge.

A further object of the invention is to provide a method for makingcertain electrodes which are particularly useful in determining thesusceptibility of metals to corrosive attack when in contact withcorrosive electrolytes.

An additional object of this invention is to provide a method of coatinga metal with an oxide which is several molecular layers thick andtightly adherent to the base metal.

Other objects and advantages of the invention will become apparent uponreading the description of the invention which follows.

According to the present invention, there is provided a method for thepreparation of oxides on metals, said method comprising the step ofimmersing the metal in a molten oxidizing salt, such as an alkali metalnitrate or alkali metal chlorate, for a period of time sufficient toform an oxide coating.

The characteristics of the individual electrodes in various environmentsand the nature of the anodic system of corrosion prevention aredisclosed herein with the aid of illustrative drawings and graphs asoutlined below:

FIGURE 1 is a schematic illustration of a typical anodic polarizationcorrosion control system.

FIGURE 2 is a graph illustrating a typical polarization curve for ametallic vessel to be protected from corro sion by contact with acorrosive electrolyte contained therein.

FIGURE 3 is a schematic diagram illustrating the apparatus which I haveutilized in observing the stability of the potential of the referenceelectrodes of the present invention by measuring the potentialdifference between the electrode under test and a calomel referencehalf-cell maintained at approximately 25 C.

FIGURE 4 is a graph in which potential is plotted against temperaturefor a platinum-plantium oxide electrode in contact with concentratedsulfuric acid.

FIGURE 5 is a graph in which potential is plotted against temperaturefor a platinum-platinum oxide electrode in direct contact with pick-1eliquor electrolyte.

FIGURE 6 is a graph in which potential is plotted against acidconcentration for certain noble metal oxide electrodes when in contactwith a sulfuric acid electrolyte maintained at 25 C.

FIGURE 7 is a graph in which potential is plotted against temperaturefor a platinum-platinum oxide electrode in direct contact with 85%phosphoric acid.

FIGURE 8 is a graph in which potential is plotted against temperaturefor a platinum-platinum oxide electrode in contact with 20% sodiumhydroxide.

FIGURE 9 is a graph in which potential is plotted against time forplatinum-platinum oxide electrode immersed in sulfuric acid solutionsmaintained at different temperatures.

FIGURE 10 is a graph in which potential is plotted against time in orderto illustrate the stability of a platinum-platinum oxide electrode inthe 85% phosphoric acid over extended periods of time.

FIGURE 11 is a graph showing a polarization curve for 1020 mild steel incontact with molten sodium hydrogen sulfate and using aplatinum-platinum oxide reference electrode.

FIGURE 12 is a graph showing a polarization curve for 304 stainlesssteel in contact with molten sodium hydrogen sulfate and using aplatinum-platinum oxide reference electrode.

Referring now to the drawings in detail and particularly to FIGURE 1, atypical anodic polarization corrosion control system generally comprisesa metallic vessel 10 or other metallic member which it is desired toprotect from the corrosive influence of a corrosive electrolyte 12 whichis in contact therewith. The anodic polarization system per se comprisesan inert electrode 14 which is suspended in the corrosive electrolyte1'2 and which is made a cathode with respect to the metallic vessel 10which is connected as the anode in a suitable electrical circuit. Theelectrical circuit which includes the cathode 14 and the vessel 10comprises a suitable source of direct current 16, electrical leads 18and 20 connected to the cathode 14 and the vessel 10 respectively, and asuitable switch 22 for opening and closing the electrical circuit.

The control of the closure of switch 22 and consequently the passage ofcurrent between the vessel 10 and the inert cathode 14 is effected bymeans of a reference electrode 24 and suitable control circuitry 26. Thereference electrodes which have previously been used have generallyrequired the use of a suitable electrolytic salt bridge, which isdesignated in FIGURE 1 by reference character 28, in order to place thereference electrodes in electrochemical communication with the corrosiveelectrolyte in the vessel 10.

As a potential of the metallic vessel 10 is varied with respect to thereference electrode 24 the susceptibility of the vessel to the corrosiveattack by the corrosive electrolyte 12 is also varied. An indication ofthe passivity of the vessel or its immunity to corrosive attack cantherefore be determined by observing the variation in the potentialdifference between the reference electrode 24 and the metallic vessel10. Since the potential of a properly functioning reference electroderemains essentially constant, variations in the potential differencebetween this electrode and the vessel 10 will be indicative of a changein the potential of the vessel 10 and hence a change in itssusceptibility to corrosive attack. The controller 26 converts thevariation in this potential difference to control signals which operatethe switch 22 so that current is passed from the electrolyte between thevessel '10 and the inert cathode 14 at such times as may be required tomaintain or rest-ore the metal of the vessel 10 to a passive state.

In FIGURE 2 of the drawings, a typical anodic polarization curve isillustrated. This curve will be characteristic of each particular vesseland electrolyte system which is under the protection of the system. Itwill be noted that over the range of potentials at which the vessel ispassive very little current is required to be passed between the vesseland the inert cathode in order to maintain the vessel at these passivepotentials. On the other hand, to pass from an active potential to therange of passivity a large amount of current must be applied to causethe potential to change through the fiade point. It should also be notedthat most electrolytic systems requiring protection will, as shown inFIGURE 2, have a relatively large range of potential over which thevessel will be passively or relatively inert (noble) with respect to theelectrolyte. For this reason, an isothermal potential deviation ofmillivolts or more can frequently be tolerated in the referenceelectrode without the occurrence of detrimental errors in thedetermination of the time and quantity of current which should bepassed.

As previously indicated, it is one of the major objects of the presentinvention to effect improvement in anodic polarization corrosion controlsystems of the type illustrated in FIGURE 1 by providing referenceelectrodes which function in a manner superior to the calomel andsilver-silver chloride electrodes which have previously been utilizedfor this purpose. In order to evaluate the extent to which a number ofmaterials might approach ideality or at laest suitability in theirability to function as reference electrodes, a great many materials havebeen tested by us in a variety of corrosive elcetrolytes which generallyinclude those most frequently encountered in chemical storage systems ofthe type where corrosion protection is most essential. The electrolyteswhich were employed in evaluating possible reference electrode materialswere: sulfuric acid, oleum, phosphoric acid, polyphosphoric acid, pickleliquor, sodium hydroxide and chromic acid. Varying concentrations andtemperatures of each of these electrolytes were utilized; and in thecase of most of the electrode materials tested, quantities of iron weredissolved in the electrolytes to determine the effect, if any, thismaterial might have upon the constancy of the potential of the materialtested as electrodes. The provision of substantial quantities of iron insolution in the eelctrolytes, in many instances, was effected for thepurpose of simulating more closely the conditions actually obtaining insituations where anodic polarization systems are utilized to prevent thecorrosion of vessels constructed of ferrous metals.

In order to accurately evaluate the variations in potential of thematerials which were tested for suitability as reference electrodes, thetesting apparatus illustrated schematically in FIGURE 3 was devised. Myprevious work prior to design of the apparatus illustrated in FIGURE 3had shown that the deterioration of agar salt bridges interposed betweena corrosive electrolyte and a reference calomel electrode for theprotection of the calomel electrode had caused the measured between thematerial under test and the calomel electrode to drift. The apparatusillustrated in FIGURE 3 virtually eliminates mixing of the salt bridgesolution and the corrosive electrolyte being used in the test of theelectrode material.

Referring to FIGURE 3, a vessel 30 constructed of glass or othersuitably inert material is used to contain a corrosive electrolyte 31utilized in the test. This vessel is provided with a stopper 32 whichcarries a thermometer 34 and a test electrode material 36, both of whichextend into contact with the corrosive electrolyte solution 31 in thevessel 30. The vessel 30 is placed on a thermostatically controlledheater 38 which facilitates the control variations of the temperature ofthe corrosive electrolyte. A calomel electrode 40 is placed incommunication with a saturated potassium chloride solution 44 containedin a suitable vessel 46. Electrically conductive communication isestablished between the corrosive electrolyte 31 and a saturatedpotassium chloride solution 44 in the vessel 46 by means of side arms 48and 50 extending from the vessels 30 and 46 respectively andcommunicating with additional saturated potassium chloride solution 52contained within a third vessel 54. Control of the flow of the corrosiveelectrolyte in a side arm 48 is achieved by means of stopcocks 56 and 58and by a vent arm 61 communicating with the side arm 48 and containing astopcock 62. A stopcock 64 is interposed in the side arm 50 to controlthe flow of saturated potassium chloride solution between the vessel 46and the vessel 54. All of the stopcocks are maintained in a greasefreecondition. The calomel reference half-cell 40 is maintained at atemperature of approximately 25 C. and the potential diiference betweenthe test electrode and the calomel reference half-cell may be measuredwith a Millivac D.C. Voltmeter (Type MV 17C), a Keithley 1;,

Electrometer (Type 610A) or other suitable measuring device.

A number of reversible electrodes comprising a metallic base with ametal oxide coating thereon were tested for suitability as referenceelectrodes in anodic polarization corrosion control systems.

These electrodes were prepared by immersing rods of platinum, rhodium,tantalum, paladium, hafnium, gold, molybdenum, tungsten and chromiumindividually in a molten salt bath for a period of time sufiicient toform an oxide coating. The oxide coatings were identified by X-ray. Theexact temperature of the molten salt was not critical in the formationof the oxide, the only prerequisite being that the salt be in the moltenstate. The

period of time required to form the oxide varied between about 0.5 andabout 12 hours, the exact time being dependent upon the particular metalbeing oxidized. The results are presented in Table A.

volts to 490 millivolts. Despite this slight temperature instability ofthe platinum-platinum oxide electrode in a sulfuric acid electrolytecontaining iron, the many other advantageous qualities of this electrodemake it one of the most suitable for use in anodic polarizationcorrosion control systems; thus, it will be perceived by reference toFIGURE 5 that the platinum-platinum oxide electrode performs well in asulfuric pickle liquor electrolyte. Its mean potential stability is alsoexcellent in 20% sodium hydroxide as indicated in FIGURE 8. Thetemperature stability of the platinum-platinum oxide electrode in 85%phosphoric acid electrolyte in which no iron was dissolved, Whilesatisfactory, is not as good as the stability of this electrode in otherelectrolytes in which it was tested. The results of potentialmeasurements made while the electrode was in direct contact with thephosphoric acid electrolyte are portrayed in the graph illustrated inFIG- URE 7. It will be noted that the temperature range over which thetest was conducted in obtaining the FIGURE 7 data was from about 22through 130 C.

In order to evaluate the long-term electrode stability ofplatinum-platinum oxide electrodes, tests were conducted to evaluatethis parameter when the electrode was placed in direct contact with 67%sulfuric acid containing no iron and maintained at 100 C.; 67% sulfuricacid containing no iron and maintained at 25 C.; and 85% phosphoric acidcontaining no iron and maintained at 25 C. The results of these testsare plotted in the graph shown in FIGURES 9 and 10. It will be notedthat in each case the stability of the potential of the electrode inthese electrolytes over extended periods of time was excellent, being inevery instance as good or better than the long-term electrode stabilityof the calomel electrode.

TAB LE A Analytical Temp, Time, Coating Method Example Metal Molten SaltC. Hours Appearance Formed Used In Analyzing Product 400 1 Dull -1 U.V.*400 l d0 U.V.* 400 1 do U.V.* 400 12 do X-ray. 400 12 do Do. 400 12 doDo. 420 O. 5 Blue. D0. 400 1 Gray Do. 400 1 do Do. 400 t d0 D0. 400 BlueD0. 400 12 Dull Do. 400 12 Gray Do. 400 12 Blue. D0. 400 12 Gray Do.

*Beckman D.U. Spectrophotometer at 230 and 260 mu.

The preferred method of preparing the platinum-platinum oxide electrodeis illustrated in Table A in which a platinum rod was dipped in fusedpotassium nitrate maintained at a temperature of about 400 C. and wasallowed to remain in the molten salt for a period of at least one hour.In the electrodes prepared by this method, the

amorphous coating was lusterless and without apparent average filmconsists of a mixture of PK) and PtO in 5 the ratio range of about 3:1to about 7:1 respectively. The film thickness has been found to varyfrom about 12 to 157 atomic layers.

In FIGURE 4 the results of potential stability tests of aplatinum-platinum oxide electrode in 67% sulfuric 70 acid containing 54parts per million iron have been portrayed in the form of a graph. Thetemperature stability of this electrode is seen to be comparable to thatof the calomel electrode. The range of measured potential over atemperature range of 100 C. is from about 390 milli- In addition to thetest of the platinum-platinum oxide electrode which yielded the resultsplotted in the graph shown in FIGURES 4, 5, 7 and 10, this electrode wasalso tested to determine the stability of its potential in phosphoricacid solutions in varying concentrations. The results obtained frommeasurements of the relationship between the platinum-platinum oxideelectrode and the reference saturated calomel electrode at approximately25 C. is tabulated in Table B. It will be seen from the data in thistable that the platinum-platinum oxide electrode is relativelyinsensitive to changes in phosphoric acid concentration and evenmaintains its potential stability in poly-phosphoric acid containingfree phosphorus pentoxide.

TABLE B Acid concentration referenced to A test was also conducted todetermine the stability of the potential of a platinum-platinum oxideelectrode in a chromic acid cleaning mixture of the type commonlyemployed in cleaning laboratory glassware. This cleaning mixtureconsisted of 92 grams of sodium dichromate dihydrate, 458 grams ofdistilled water, and 800 ml. of concentrated sulfuric acid (96%). Thetime required for the electrode to become stabilized in the chromic acidcleaning mixture was evaluated and the results of this evaluation aretabulated in Table C below.

TABLE C.PLATINUM PLATINUH OXIDE ELECTRODE IN CHROMI'C ACID CLEANINGMIXTURE referenced to.

Time, minutes: calornel, in millivolts It will be perceived that theplatinum-platinum oxide electrode came readily to a stable value andretained this over an extended period of time. This result confirms theextended time stability of the platinumplatinum oxide electrode observedin the sulfuric acid electrolyte and represented by the graph depictedin FIG- URE 9.

In addition to the platinum-platinum oxide electrode, a rhodium-rhodiumoxide electrode was prepared in the manner similar to the preparation ofthe platinum-platinum oxide electrode. A strip of rhodium metal wasdipped in a bath of fused potassium nitrate maintained at a temperatureof about 400 C. The rhodium strip was maintained in contact with the hotsalt for a period exceeding twelve hours. The type of oxide which isformed by this procedure was determined by X-ray defraction to be Rh203.

Upon testing the rhodium-rhodium oxide electrode in sulfuric acidelectrolytes of varying temperature and concentration, a behaviorsubstantially identical to that exhibited by the platinum-platinum oxideelectrode resulted. Temperature stability of the electrode was verygood; however, in the case of both the rhodium-rhodium oxide andplatinum-platinum oxide electrodes, the potential of the electrodesremained substantially constant over a range of lower concentrations ofthe sulfuric acid electrolyte then changed sharply at a concentration ofapproximately 96% of the acid and then once again assumed stability inconcentrations of from approximately 100% to 115% sulfuric acid. Thesetests of electrode potential stability against sulfuric acidconcentration indicate that the rhodium-rhodium oxide andplatinum-platinum oxide elec-' trodes can be used to excellent advantagein a corrosive sulfuric acid electrolyte of less than 96% acidconcentration or of greater than 100% acid concentration (oleum).Between these two ranges of acid concentration, however, a factor mustbe introduced to compensate for the change in electrode potential withchanges in acid concentration. A preferred use of these two noble metaloxide electrodes would therefore appear to be in oleum service andsulfuric acid service where the concentration of the sulfuric acid isless than 96%.

In addition to the electrodes made from noble metals rhodium andplatinum, a gold-gold oxide electrode was 'also fabricated in a similarmanner and appeared to possess electrode characteristics quite similarto that of the rhodium-rhodium oxide and platinum-platinum oxideelectrodes. Thus, in FIGURE 6, it may be observed that the potential ofthe gold-gold oxide electrode also is quite stable for sulfuric acid oroleum concentrations of less than 96% and greater than 100%respectively. The stability of the gold-gold oxide in oleum is evidencedby the data tabulated in Table D.

It is apparent that the electrode maintains a substantially constantpotential as the percentage of free sulfur trioxide in the oleumelectrolyte is varied between 15% and 33%. This stability of the noblemetal oxide electrode in oleum solutions is especially desirable in Viewof the widespread use of this electrolyte in various chemicalmanufacturing and processing industries.

One of the most desirable characteristics of the goldgold oxideelectrode is its ability to become readily stabilized in most of theelectrolytes in which it was tested. In this respect the gold-gold oxideelectrode was superior to either the platinum-platinum oxide electrodeor the rhodium-rhodium oxide electrode. Typical data showing the eifectof aging on the potential of the gold-gold oxide electrode are tabulatedin Table E which shows the effect of maintaining the electrode incontact with an oleum electrolyte containing from 30% to 33% free sulfurtrioxide over a period of 24 hours.

TABLE E referenced to Time, hours: calomel, in millivolts 0.25 830 4.75830 8.25 850 24.0 840 The data tabulated in Table E indicate that within15 minutes of its being placed in contact with the oleum electrolyte thegold-gold oxide electrode has attained the potential which it will thenmaintain for the next 24 hours. The excellent stabilization rate of thiselectrode is believed to be superior to the rate of stabilization of anyof the other materials tested and found to be suitable for use asreference electrodes. In tests of the gold-gold oxide electride in aphosphoric acid electrolyte, the electrode demonstrated very littlepotential fluctuation as the concentration of the phosphoric acid wasvaried. The results of these tests are tabulated in Table F.

TABLE F Acid concentration, referenced to Percent H PO calomel, inrnillivolts 85.9 96.3 100 The results which are set forth in Table Frepresent only approximate values obtained for the goldgold oxideelectrode because of the small size of the electrode used; however, thedata are considered sufiiciently accurate to show that the electrodepotential is essentially independent of the concentration of thephosphoric acid electrolyte.

Like the platinum-platinum oxide electrode the goldgold oxide electrodewas also tested for potential stability over extended periods of timewhen placed in contact with a chromic acid cleaning mixture. Thecomposition of the mixture was the same as that described above inconnection with the description of the platinum-platinum oxide tests.The data obtained in conducting this test are tabulated in Table G.

9 TABLE G referenced to Time, minutes: calomel, in millivolts 1360 It isagain apparent that the gold-gold oxide rapidly attains, and thencontinues to maintain, an excellent stability over extended periods oftime.

Another extremely desirable property of the noble metal oxide electrodeswhich should be mentioned is their complete immunity to attack by thecorrosive electrolytes which were utilized. No tendency towardssolubility in the electrolytes Was observed.

In addition to their usefulness in sulfuric acid, phosphoric acid, andchromic acid, the noble metal oxide electrodes were found to be usefulreference electrodes in a system used to protect metallic vessels fromcorrosive attack by molten salt solutions and particularly molten acidsalts such as sodium hydrogen sulfate. Since substantially all of thewater is normally driven off of the salts in the molten state, noproblem of variation of electrode potential with the electrolyteconcentration is encountered in this application.

Because of the high temperatures encountered in the use of anodicpolarization systems to protect metals against attack by molten salts,it is not possible to evaluate the stability of the noble metal-noblemetal oxide electrodes by comparison with the standard calomelelectrode. At the temperatures at which many salts are molten, the saltbridge electrolyte solution would be caused to boil. However,polarization curve and corrosion test data obtained using theplatinum-platinum oxide electrode as the reference electrode clearlyindicate the suitability of the noble metal-noble metal oxide electrodesfor this use. Thus, in FIGURES 11 and 12, the polarization curves for1020 mild steel and for 304 stainless steel in contact with moltensodium hydrogen sulfate and using a platinumplatinum oxide referenceelectrode each indicate the existence of a passive potential region ofminimum current density.

Table H shows the corrosion rates of both unprotected and anodicallypolarized 1020 mild steel and 304 stainless steel coupons in moltensodium hydrogen sulfate at 420 F. The platinum-platinum oxide electrodewas used as the reference electrode in an anodic polarization corrosioncontrol system of the type herein before described and illustrated inFIGURE 1.

Another type of metal-metal oxide electrode was proved, by the testwhich I have conducted, to possess properties indicating its suitabilityfor use as a reference electrode in the anodic polarization corrosioncontrol systems. This was a molybdenum-molybdenum oxide electrode. In atest of this material the EMF. relationship between themolybdenum-molybdenum oxide electrode and a saturated calomel referenceelectrode maintained at 25 C. was determined while themolybdenum-molybdenum oxide electrode was in contact with phosphoricacid solutions of diiferent concentrations. As shown by Table 1 below,the EMF. of this electrode with respect to calomel is suitablyindependent of the strength of the phosphoric acid solution as the acidconcentration is varied from TABLE I Acid concentration referenced topercent H PO calomel, in millivolts 85.9 96.3 115.0 220 Themolybdenum-molybdenum oxide electrode was also evaluated for its abilityto quickly attain equilibrium and to maintain its stability over anextended period of time. Typical data showing the eflect of aging on thepotential of the molybdenum-molybdenum oxide electrode are tabulated inTable I. The tests were conducted with the electrode in contact with a115% phosphoric acid solution maintained at 25 C.

TABLE II referenced to Time, hours: calomel, in millivolts 0 190 /z 200As indicated by the data set forth in Table I, the molybdenum-molybdenumoxide electrode attains equilibrium rapidly and maintains a relativelyconstant potential over extended periods of time. In the test ofextended stability, which was conducted, it Was also observed that thiselectrode is not appreciably attacked by the phosphoric acid solutionsranging in concentration from 86% to 115%.

1 Approximately 20-hour tests. 2 Pt-PtO reference electrode (420 F.). 3For maintaining polarization near close of test.

In referring to Table H it will be noted that anodic protection reducedthe liquid phase corrosion rate of mild steel from 924 to 2.3milli-inches per year or 99.8%, and reduced the corrosion rate ofstainless steel from 81 to 5.4 milli-inches per year or 93.3%. Theseresults clearly demonstrate the eifectiveness of the noble metal-noblemetal oxide electrodes as reference electrodes for use in anodicpolarization corrosion control systems for protecting steel vesselsagainst corrosion by molten salts.

A chromium-chromium oxide electrode was also prepared in the mannerpreviously disclosed. This electrode is very useful in causticsolutions. Table K shows the stability of the chromium-chromium oxideelectrode in 50% sodium hydroxide solution when the solution was heatedto 100 C. It should also be noted that the chromium-chromium oxide wasnot visibly attacked by this concentrated sodium hydroxide solution,even at the elevated temperatures.

Ill

TABLE Ill-EFFECT OF TEMPERATURE ON E.M.F. OF CHROMIUM-CI'IROMIUM OXIDETempeature, E.M F., mv. Time, Hours 23 +545 (active) 11 23 +520 23 +52531 +530 23% +540 23% +550 23% +570 231/; so +600 23% +635 23% 1 Tosaturated calomcl at approximately 25 C.

A tungsten-tungsten oxide electrode was prepared in the manner aspreviously disclosed. This electrode was then tested in a 50% sodiumhydroxide solution and the temperature was varied over a period of time.The results are shown in Table L.

TABLE L.EFFEGT OF TEMPERATURE ON E.M.F. OF

TUNGSTEN IUNGSTEN OXIDE ELECTRODE IN 50 PER CENT CAUSTIC Temperature,BALE, mv. Time, Hours 135 l, 120 (active) 0 130 1, 120 1, 140 110 l, 125105 l, 125 100 1, 125 90 1, so 1, 70 l, 100 60 1, 100 50 1, 100 45 l,040 40 1, 010 27 l, 020 3% 27 950 45 1 To saturated calomel atapproximately 25 C.

These data show: (1) the was essentially constant at minus 1120millivolts (noble) throughout the temperature range 50 to C.; (2) thevaried slightly at 27 C. (minus 950 to minus 1020 millivolts); and (3)the shifted only to minus 1120 millivolts as the temperature wasincreased from 27 C. to 50 C.; thus, the electrode is reasonably stableat room temperature, shifts only slightly as the temperature isincreased and is essentially constant over a wide range of elevatedtemperatures.

A hafnium-hafnium oxide electrode was prepared in the manner previouslydescribed and this electrode was tested in a 50% sodium hydroxidesolution, over a temperature range of 25 to 80 C., and found to performin a very satisfactory manner.

Tantalum metal was oxidized according to the method of this inventionand the oxide formed was determined by X-ray to be the beta-tantalumoxide. This electrode was then tested in a 50% sodium hydroxide solutionover a temperature range varying from 25 to 95 C. and found to be asuitable reference electrode.

A nickel-nickel oxide electrode, prepared by the method of thisinvention, gives a reference electrode suitable for use in strongcaustic solution.

In addition to the metal oxides previously disclosed, the oxides ofmanganese, cobalt, and scandium are included within the scope of thisinvention.

From the foregoing working examples, it will be perceived that I haveextensively tested and evaluated a great number of materials todetermine their suitability for use as reference electrodes. While thesetests and evaluations have been directed primarily to the determinationof materials which possess properties which endow them with superiorattributes when utilized as reference electrodes limits of the presentinvention.

in anodic polarization corrosion control systems, it will be apparentthat many of the same properties which characterize reference electrodesin such corrosion control applications also must characterize anyreference electrode when employed in an application or use whichrequires it to be placed in contact with a corrosion electrolyte. Thesolid nature of many of the materials which have proved suitable forsuch use enables them to be usefully employed in applications wheresubstantial mechanical strength and ease of installation are importantfactors. Moreover, the excellent temperature and concentration stabilityof most of the electrodes discussed hereinbefore indicates that theypossess the properties which are most essential in various analyticaltechniques where a reference electrode of constant potential isrequired.

Many of the electrode materials which were tested and which werebelieved to be sufiiciently good materials for use as referenceelectrodes appeared to retain their attractive properties in both acidicand basic electrolytes of varying types and concentrations. Others ofthe electrodes showed definitely better stability in some of theelectrolytes than in others. In the relatively few instances where thematerials did not exhibit a constant potential over the entireconcentration range of the electrolytes in which they were tested, avery good stability of these electrodes within certain specific rangesmay be advantageously utilized in a corrosion control situation wherethe range over which the corrosive electrolyte may vary is known inadvance. It is believed that the preferred application of the severalelectrodes will be manifest from the foregoing description of theinvention and that it is unnecessary to here summarize the properties ofthe several electrodes which make one electrode or one group ofelectrodes preferable for use in certain electrolytes or in certainconcentrations of electrolytes and other electrodes or groups ofelectrodes preferable for general and widespread usage where theconcentration temperatures and chemical constitution of the electrolytesare not known with certainty in advance of the installation of thecorrosion control system.

In describing the preparation of the metal oxides of this invention byway of examples, certain temperatures and times were given. It must beunderstood that the exact temperature of the molten salt is not criticalin the formation of the oxides, the only prerequisite being that thesalt be in the molten state. Also, the time required to form the oxidevaried between about 0.5 and about 12 hours, the exact time beingdependent upon the particular metal being oxidized.

Although a wide variety of applications and usages of the electrodematerials which I have for the first time evaluated for use as referenceelectrodes will occur to electrochemists and others skilled in the art,it is our intention that the novel principles and concepts hereindisclosed shall be the criteria establishing the bounds and Insofar asminor modi fications and innovations are evolved by those skilled in theart for making further use of the novel principles and concepts hereindisclosed, it is my intention that such modifications and innovations beconsidered to fall within the sphere and scope of this invention exceptinsofar as the same may be necessarily limited by the appended claims orreasonable equivalents thereof.

What is claimed is:

1. The method of forming a metal-metal oxide electrode which comprises:

immersing a metal consisting essentially of a metal selected from thegroup consisting of platinum, rhodium, tantalum, palladium, hafnium,gold, molybdenum, tungsten, scandium, manganese, and mixtures thereof ina molten bath consisting essentially of a salt selected from the groupconsisting of alkali metal nitrates and alkali metal chlorates for aperiod of time suificient to form an oxide coating.

4. The method of claim 1 wherein the metal immersed 5 in the molten saltis gold.

5. The method of claim 1 wherein the metal immersed in the molten saltis hafnium.

6. The method of claim 1 wherein the metal immersed in the molten saltis tantalum.

l0 7. The method of claim 1 wherein the metal immersed in the moltensalt is molybdenum.

8. The method of claim 1 wherein the metal immersed in the molten saltis tungsten.

14 10. The method of claim 1 wherein the metal immersed in the moltensalt is manganese.

References Cited by the Examiner UNITED STATES PATENTS 1,247,086 11/1917Crowe 1486.11 1,879,701 9/1932 Marino 148-611 2,347,564 4/ 1944Koestel'ing 1486.11 2,431,986 12/1947 Clingan 148-6.11 2,479,979 6/1949Spence et a1 148-6.11 X

FOREIGN PATENTS 3 16,422 11/ 1956 Switzerland.

9. The method of claim 1 wherein the metal immersed 15 RALPH KENDALL,Primary Examiner in the molten salt is scandium.

1. THE METHOD OF FORMING A METAL-METAL OXIDE ELECTRODE WHICH COMPRISES:IMMERSING A METAL CONSISTING ESSENTIALLY OF A METAL SELECTED FROM THEGROUP CONSISTING OF PLATINUM, RHODIUM, TANTALUM, PALLADIUM, HAFNIUM,GOLD, MOLYBDENUM, TUNGSTEN, SCANDIUM, MAGNANESE, AND MIXTURES THEREOF INA MOLTEN BATH CONSISTING ESSENTIALLY OF A SALT SELECTED FROM THE GROUPCONSISTING OF ALKALI METAL NITRATES AND ALKALI METAL CHLORATES FOR APERIOD OF TIME SUFFICIENT TO FORM AN OXIDE COATING.